build: bump minimum Go version to 1.18, as a dependency in ./tools requires it
build: bump minimum Go version to 1.18, as a dependency in ./tools requires it

https://app.circleci.com/pipelines/github/zrepl/zrepl/6085/workflows/bf5b11f2-8dc4-40a2-bb7a-fcf3cf8205d4/jobs/42340

  ...
  build github.com/golangci/golangci-lint/cmd/golangci-lint: cannot load io/fs: cannot find module providing package io/fs
  go install github.com/wadey/gocovmerge
  go: downloading github.com/wadey/gocovmerge v0.0.0-20160331181800-b5bfa59ec0ad
  go: extracting github.com/wadey/gocovmerge v0.0.0-20160331181800-b5bfa59ec0ad
  go install golang.org/x/tools/cmd/goimports
  # golang.org/x/mod/module
  ../../go/pkg/mod/golang.org/x/mod@v0.6.0/module/module.go:147:5: undefined: errors.As
  note: module requires Go 1.17
  go install golang.org/x/tools/cmd/stringer
  # golang.org/x/tools/go/internal/gcimporter
  ../../go/pkg/mod/golang.org/x/tools@v0.2.0/go/internal/gcimporter/iimport.go:520:9: undefined: constant.Make
  ../../go/pkg/mod/golang.org/x/tools@v0.2.0/go/internal/gcimporter/iimport.go:616:9: undefined: constant.Make
  note: module requires Go 1.18
  go install google.golang.org/grpc/cmd/protoc-gen-go-grpc
  go: downloading google.golang.org/grpc v1.46.2
  go: extracting google.golang.org/grpc v1.46.2
  go: downloading google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0
  go: extracting google.golang.org/grpc/cmd/protoc-gen-go-grpc v1.1.0
  go install google.golang.org/protobuf/cmd/protoc-gen-go

  Exited with code exit status 123

zrepl/zrepl,zrepl/zrepl